3o6 Whether the Cryfiatlme Lens he the InJlnimeiU 



nearer approach of the cryftalline to a fpherical form ; and he could imagine no other power 

 capable of producing this change than a niufcularity of part or the whole of its capfule. 



But on clofely examining, with tlie naked eye, the cryrtalline from an ox turned out of its 

 capfule, he difcovercd a (Iruclure which he thinks faflicient to remove all the difficulties 

 with wliich this branch of optics has long been obfcurcd. The cryftalline lens of the ox is 

 an orbicular, convex, tranfparent body, compofed of a confiderable number of fimilar coats, 

 of which the exterior clofely adhere to the interior, tach of tbefe coats conCfts of fix 

 mufcles, intermixed with a gelatinous fubftance, and attached to f:x membranous tendons. 

 Three of the tendons are anterior, tliree pofterior ; their length is about two-thirds of the 

 femi-diaineter of the coat ; their arrangement is that of three equal and equiilillant rays, 

 meeting in the axis of the cryftalline ; one of the anterior is direfteJ towards the outer 

 angle of the eye, and one of the pofterior towards the inner angle ; fo that the pofterior 

 are placed oppofite to the middle of the interftices of the anterior ; and planes pafling 

 through each of the fix, and thro.ugh the axis, would mark on either fuiface fix regular equi- 

 diftant rays. The mufcular fibres arifc from both fi;les of each tendon ; they diverge till 

 they reach the greateft circumference of the coat; and having pafled it, they again converge 

 till they are attached refpeclively to the fides of the neareft tendons of the oppofite furface. 

 The anterior or pofterior pottion of the fix viewed together exhibits the appearance of three 

 penniform-radiated mufcl-es. The anterior tendons of all the coats are fituated in the 

 lame planes, and the poderior ones in the continuations of thefe planes beyond the axis. 

 Such an arrangement cf fibres can be accounted for on no other fuppofition than that of 

 roufcularity. This mafs is enclofed in a ftrong membranous capfule, to which it is loofely 

 connedled by minute vtflels and nerves; and the connexion is more obfervable near its 

 greateft circumference. Between the mafs and its capfule is found a confiderable quantity 

 of an aqueous fluid, the liquid of the cryftalline. 



He conceives, theiefore, that when the will is exerted to view an objeft at a fmall diftance, 

 the influence of the mind is conveyed through the lenticular ganglion formed from branchea 

 of the tliird and fifth pairs of nerves, by the filaments perforating the fclerotica to the or- 

 biculus ciliaris, which may be confidered as an annular plexus of nerves and veflels ; and 

 thence, by the ciliary proccffLS, to the niufcle of the cryftalline ; which, by the contra£lion 

 of its fibres, becomes more convex, and colle£ls the diverging rays to a focus oa the retina. 

 The difpofiti n of fibres in each coat is admirably adapted to produce this change j for fince 

 the Icaft furface that can contain a given bulk, is that of a fphere (Simpfon's Fluxions, p. 486), 

 the contraftion of any furface muft bring its contents nearer to a fpherical form. The liquid 

 of the cryftalline fcems to ferve as a fynovia in facilitating the motion, and to admit a fuf- 

 ficient change of the mufcular part, vi^ith a fmaller motion of the capfule. 



To afcertain whether thefe fibres can produce an alteration in the form of the lens fuf- 

 ficiently great to account for the known effefts, this author ftates, that the diameter of the 

 tryftahine of the ox is 700 thoufandths of an inch, the axis of its anterior fegment 225 ; 

 of its pofterior 350. In the atmofphere it colledls parallel rays, at the diftance of 235 

 thoufandths. From thefe data he finds, by means of Smith's Optics, article 366, and a 

 quadratic, that its ratio of refraflion is as loooo to 6574. Haukfbee makes it only as 

 loooo to 6832,7 ; but we cannot depend on his experiment, fince he fays, that the image 

 oi the candle which he viewed was enlarged and diftorted ; a circumftance that he does not 



explain. 



